In recent years in response to the ever-increasing miniaturization and densification of microelectronic circuit elements as used in, for example, semiconductor integrated circuits, microlithography apparatus and methods have been developed that employ a “soft X-ray” (SXR) beam as a lithographic energy beam. SXR microlithography (also termed “extreme ultraviolet” or “EUV” microlithography) offers prospects of achieving better resolution of fine pattern elements than currently obtainable using conventional optical microlithography (i.e., microlithography performed using “vacuum” ultraviolet, or VUV, wavelengths). SXR radiation has a wavelength generally within the range of 11 nm to 14 nm, which is significantly shorter than the wavelength of VUV radiation (about 150 nm or greater). In other words, current optical microlithography technology is compromised by diffraction limits, which prevent obtaining ever greater resolution (e.g., see Tichenor et al., Proceedings SPIE 2437:292, 1995). EUV microlithography has been hailed as the “microlithography of the future,” capable of achieving resolutions of about 70 nm and smaller, which cannot be achieved using optical microlithography.
With SXR wavelengths, the refractive index of substances is extremely close to unity. As a result, conventional optical elements for achieving refraction and/or reflection of optical wavelengths cannot be used. Instead, grazing-incidence mirrors or multilayer-film mirrors typically are used. Grazing-incidence mirrors achieve total reflection with a refractive index of slightly less than unity, and multilayer-film mirrors achieve a high overall reflectivity by passing weakly reflected light from each layer through multiple phase-matched convolutions. For example, a reflectivity of 67.5% can be obtained of a normal incident beam having a wavelength of about 13.4 nm using a reflective mirror comprising a Mo/Si multilayer film, in which molybdenum (Mo) layers and silicon (Si) layers are laminated in an alternating manner. A reflectivity of 70.2% can be obtained of a directly incident beam having a wavelength of about 11.3 nm using a reflective mirror comprising a Mo/Be multilayer film, in which Mo layers and beryllium (Be) layers are laminated in an alternating manner. For example, see Montcalm, Proceedings SPIE 3331:42, 1998.
Conventional SXR microlithography apparatus comprise a SXR source, an illumination-optical system, a reticle stage, an imaging-optical (projection-optical) system, and a substrate stage. The SXR source can be a laser-plasma source, a discharge-plasma source, or a synchrotron-radiation source. The illumination-optical system comprises grazing-incidence mirrors each having a respective reflective surface that reflects SXR radiation that is obliquely incident to the reflective surface, multilayer-film mirrors each having a reflective surface are formed by a multilayer film, and a filter that transmits only SXR radiation of a specified wavelength. Thus, the reticle is illuminated by SXR radiation having a desired wavelength.
Since no known substances are transparent to SXR radiation, the reticle is a so-called “reflective reticle” rather than a conventional transmission-type reticle. The imaging-optical system comprises multiple multilayer-film mirrors, and is configured to form an image, of the irradiated region of the reticle, on the substrate (e.g., semiconductor wafer) to which a layer of a suitable resist has been applied. Thus, the image is transferred to the layer of resist. Since SXR radiation is absorbed and attenuated by the atmosphere, the SXR light path in the microlithography apparatus normally is maintained at a certain vacuum (e.g., 1×10−5 Torr or less).
As noted above, the imaging-optical system comprises multiple multilayer-film mirrors. Since the reflectivity of a multilayer-film mirror is not 100 percent, the imaging-optical system desirably consists of as few such mirrors as possible to minimize light loss. Thus far, imaging-optical systems comprising four multilayer-film mirrors (e.g., Jewell and Thompson, U.S. Pat. No. 5,315,629; and Jewell, U.S. Pat. No. 5,063,586) and six multilayer-film mirrors (e.g., Williamson, U.S. Pat. No. 5,815,310) have been reported. Unlike refractive optical systems through which a light flux propagates in one direction, reflective optical systems are characterized by the doubling back of the light flux on itself within the optical system. Hence, it is difficult to obtain a large numerical aperture (NA) due to restrictions such as avoiding eclipsing the light flux with the mirrors.
Whereas a NA of no more than about 0.15 can be obtained in a four-mirror imaging-optical system, it is possible for a six-mirror optical system to have an even greater NA. Normally, an even number of mirrors is used in the imaging-optical system so that the reticle stage and the substrate stage can be situated on opposite sides of the optical system. Since the imaging-optical system must correct aberrations using a limited number of surfaces, each of the mirrors typically has an aspherical profile, and a ring-field imaging scheme is used in which aberrations are corrected only in the proximity of a desired lateral displacement from the optical axis. To transfer the entire reticle pattern onto the substrate, exposure is performed while scanning the reticle stage and the substrate stage at respective velocities that differ from each other according to the magnification ratio of the imaging-optical system.
Imaging-optical systems, as discussed above, for use in SXR microlithography apparatus are so-called “diffraction-limited” optical systems. These optical systems cannot achieve the performance levels for which they were designed unless wavefront aberrations can be minimized adequately. A guideline for tolerances of wavefront aberration in a diffraction-limited optical system is Marechal's standard, in which the root-mean-square (RMS) departure of the wavefront from a reference sphere that is centered on the diffraction focus does not exceed λ/14, wherein λ is wavelength. Born and Wolf, Principles of Optics, 7th edition, Cambridge University Press, 1999, p. 528. These are the conditions for obtaining 80% or more of the Strehl intensity (the ratio of maximum point-image intensities in an optical system with aberrations to maximum point-image intensities in an optical system with no aberrations). Imaging-optical systems in actual SXR microlithography apparatus are configured to produce aberrations even lower than this.
In the SXR microlithography techniques currently under vigorous research and development, exposure “light” is used having a wavelength primarily in the range of 13 nm to 11 nm. With respect to wavefront error (WFE) in an optical system, the form error (FE) allowed for each individual mirror is given by Equation (1):FE=(WFE)/2/m1/2 (RMS)  (1)In Equation (1), “m” is the number of mirrors that make up the optical system, and WFE is divided by two because wavefront error is double the form error. This is because both incident light and reflected light in a reflective optical system are subject to the effects of each respective form error.
The form error (FE) allowed for each individual mirror in a diffraction-limited optical system is given by Equation (2), relative to wavelength λ and number of mirrors m:FE=λ/28/m1/2 (RMS)  (2)In the case of a 4-mirror imaging-optical system, this value is 0.23 nm (RMS) at a wavelength of 13 nm and 0.20 nm (RMS) at a wavelength of 11 nm. In the case of an optical system comprising 6 mirrors, this value is 0.19 nm (RMS) at a wavelength of 13 nm and 0.16 nm (RMS) at a wavelength of 11 nm.
Unfortunately, a high-precision aspherical mirror satisfying the foregoing criteria is extremely difficult to manufacture. This is the main reason why a practical SXR microlithography apparatus has not been realized yet. The fabrication accuracy achievable to date for an aspherical mirror is about 0.4 to 0.5 nm (RMS). Gwyn, Extreme Ultraviolet Lithography White Paper, EUV LLC, p. 17, 1998. Consequently, fabrication and design techniques for aspherical surfaces used in mirrors in imaging optical systems must be improved substantially in order to achieve a practical SXR microlithography apparatus that exhibits higher resolution than obtainable with current optical lithography.
Producing acceptable multilayer-film mirrors requires: (a) an ability to form an acceptable aspherical surface on a mirror substrate, (b) an ability to form a multilayer-film coating on the reflective surface of the mirror, and (c) an ability to perform minute corrections to the surface of the multilayer film as required. With respect to the latter, breakthrough technology recently was reported by Yamamoto who disclosed a method by which sub-nanometer form errors were correctable by etching away the surface of a multilayer-coated mirror one layer at a time (Yamamoto, 7th International Conference on Synchrotron Radiation Instrumentation, Berlin, Germany, Aug. 21-25, 2000, POS2-189). The principles of this method are explained with reference to FIGS. 8(a)-8(c).
FIG. 8(a) depicts a region of a multilayer-film surface consisting of three “layer sets” or “lamina sets.” Each lamina set consists of a layer of substance A and a layer of substance B and has a thickness (“period length”) denoted by “d”. Note that the layers are formed alternatingly. FIG. 8(b) depicts the region of FIG. 8(a), but with one lamina set removed from the surface of the multilayer film. In FIG. 8(a), the optical path length for one lamina set of thickness d, relative to a light beam incident perpendicularly to the surface of the multilayer film, is expressed by OP=nAdA+nBdB, wherein dA and dB represent the thicknesses of the respective layers, and dA+dB=d. The refractive indices of the substances A, B are denoted by nA and nB, respectively.
In FIG. 8(b), the optical path length of the portion, having thickness d, from which the topmost lamina set has been removed is expressed by OP′=nod, wherein no represents the refractive index of a vacuum, and no=1. Thus, by removing the topmost lamina set of the multilayer film, the optical distance through which an incident beam must propagate is changed. This is optically equivalent to correcting the surface shape by the magnitude of the change in the optical path length.
The change in optical path length (i.e., change in surface shape) is expressed by Δ=OP′−OP. Since the refractive index of a substance is near unity (1) in the SXR wavelength band, Δ is very small, which facilitates precision corrections of the surface profile of a multilayer-film mirror using this method. For example, consider a case in which a Mo/Si multilayer-film mirror is used at a wavelength of 13.4 nm. At perpendicular incidence, d=6.8 nm, dMo=2.3 nm, and dSi=4.5 nm. The refractive indices at this wavelength are nMo=0.92 and nSi=0.998. Calculating the change in optical path length using these figures yields OP=6.6 nm, OP′=6.8 nm, and Δ=0.2 nm. Thus, a surface-shape correction equivalent to 0.2 nm can be accomplished by processing the surface of the multilayer film sufficiently to remove a 6.8 nm-thick lamina set.
Since the refractive index of the Si layer is near unity (1), any change in the optical path length of a Mo/Si multilayer-film mirror achieved by removing one lamina set results primarily from the removal of the Mo layer. In other words, the presence or absence of the respective Si layer has virtually no effect. Consequently, when removing a surficial lamina set from a multilayer-film mirror, there is no need to control the thickness of the respective Si layer accurately. In this example, the Si layer was 4.5 nm thick; layer-removal processing could have been stopped in the middle of the Si layer. In other words, surface-profile correction can be performed in 0.2 nm units by performing local “milling” of the multilayer-film surface at a precision of several nm.
In the implementation of this surface-profile-correction method, after fabricating a multilayer-film mirror, the reflected wavefront produced by the mirror is measured and evaluated. Based on the results of the measurements, a predetermined amount of milling of selected loci on the surface is performed to remove one or more lamina sets as required to make identified corrections in the reflected wavefront.
The index of reflection of a multilayer-film mirror increases with corresponding increases in the number of lamina sets. This trend is applicable up to a threshold number of lamina sets, beyond which the reflectivity is saturated and remains constant as additional lamina sets are added. If the mirror is formed initially with a number of lamina sets sufficient to achieve a saturated index of reflection, the mirror will not exhibit any change in the index of reflection if subjected to local surficial milling of one or a few lamina sets.
Surficial milling of the mirror surface must be performed at high accuracy and precision. I.e., a milling apparatus must be able to control the amount of multilayer film removed from the selected loci with a high degree of accuracy and precision in order to achieve the desired correction. Exemplary conventional methods for removing surficial lamina sets are wet-etching methods using acid or alkaline solution or “sandblasting” methods using, e.g., metal or ceramic dust. Unfortunately, it is very difficult with any of these methods to achieve a desired level of control over the amount of material actually removed. In addition, removal of multiple lamina sets from a location on the surface of the multilayer film should be performed in a manner yielding a gradation of number of lamina sets actually removed, as shown in FIG. 8(c), to ensure smooth corrections to reflective wavefront errors. This is because the amount of phase shift to be corrected changes with the amount of the multilayer film actually removed, and the amount of phase shift to be corrected differs across the surface of the multilayer-film mirror. Since the amount of milling to be implemented for this correction is from a few nm to several tens of nm, the distribution of the amount actually milled must be achieved with very high accuracy and precision.